Method for encoding and decoding images according to distinct zones, encoding and decoding device, and corresponding computer programs

ABSTRACT

A method and device for encoding an image divided into blocks. The image contains two separate zones. The method implements the following: only in the event where the current block pertains to one of the zones of the image: encoding the current block using a prediction, the current block being predicted using a previously encoded and then decoded block located in the other zone of the image, the blocks of the other zone having been previously encoded and then decoded, and encoding information indicating the application of the prediction; and for any decoded block pertaining to the other zone of the image, storing the decoded block data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 16/628,470, filed Jan. 3, 2020, which is a Section 371 National Stage Application of International Application No. PCT/FR2018/051580, filed Jun. 28, 2018, published as WO 2019/008254 A1 on Jan. 10, 2019, not in English, the contents of which are incorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention pertains generally to the field of image processing, and more precisely to the coding and to the decoding of parameters of digital images, whether these digital images are fixed or form part of a sequence of digital images.

The coding/decoding of such image parameters applies notably to images arising from at least one video sequence comprising:

-   -   images arising from one and the same camera and following one         another temporally (coding/decoding of 2D type),     -   images arising from various cameras oriented according to         different views (coding/decoding of 3D type),     -   corresponding texture components and depth components         (coding/decoding of 3D type),     -   images obtained by projection of a 360° video,     -   etc. . . . .

The present invention applies in a similar manner to the coding/decoding of parameters of images of 2D or 3D type.

The invention can notably, but not exclusively, apply to the video coding implemented in the current AVC (English abbreviation of “Advanced Video Coding”) and HEVC (English abbreviation of “High Efficiency Video Coding”) video coders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc), and to the corresponding decoding.

BACKGROUND OF THE INVENTION

The current video coders (MPEG, H.264, HEVC, . . . ) use a block-wise representation of the video sequence. The images are split up into blocks, which might be split up again, for example in a recursive manner as in the HEVC standard.

For a current block to be coded, the image parameters associated with this block are coded in the form of bits with the aid of an adapted coding scheme implemented by a coder, such as for example an entropy coder whose aim is to code these parameters without loss.

Such parameters are for example:

-   -   the residual prediction coefficients of the pixels of the         current block,     -   the mode of prediction of the current block (Intra prediction,         Inter prediction, default prediction carrying out a prediction         for which no information is transmitted to the decoder (“in         English “skip”)),     -   information specifying the type of prediction of the current         block (orientation, reference image, . . . ),     -   the type of splitting of the current block,     -   the motion information of the current block if necessary,     -   etc.

The bits obtained after entropy coding are written into a data signal which is intended to be transmitted to the decoder.

Once the coded-data signal has been received by the decoder, the decoding is done image by image, and for each image, block by block. For each block, the bits representative of the image parameters associated with the block are read, and then decoded with the aid of a decoding scheme implemented by a decoder.

For each image type or format considered, a specific coding is implemented. Thus for example, the AVC and HEVC coders/decoders are adapted to code/decode 2D images arising from one and the same camera and following one another temporally.

A coding of the aforementioned type generally uses, for a current image considered, a prediction of the current block.

Various prediction modes exist, such as for example:

-   -   the Intra prediction mode which consists in predicting the         pixels of the current block with respect to pixels neighboring         those of the current block, these neighboring pixels having         already been coded and then decoded and possibly being situated         in various predetermined directions, for example thirty-five in         the HEVC standard,     -   the Inter prediction mode which consists in predicting certain         coding information associated with the current block, such as         for example:         -   the temporal motion vector of the current block with respect             to a temporal motion vector associated with a block of             reference pixels,         -   the pixels of the current block with respect to the pixels             of a block of reference pixels which is pointed at by the             temporal motion vector and is situated in an image other             than the current image to which the current block belongs.     -   the “Intra Block Copy” (IBC) prediction mode, such as described         in the document: SIP(2015), vol. 4, page 1 to 12, 2015,         Industrial technology advances, “Overview of HEVC extensions on         screen content coding” by Shan Liu, Xiaozhong Xu, Shawmin Lei         and Kevin Jou, which makes it possible to predict the current         block through a block belonging to the same image, in a         previously coded and decoded part of the latter.

These prediction modes are set into competition and the prediction mode which optimizes a coding performance criterion, such as for example the bitrate-distortion criterion, is selected for the prediction of each block of the current image.

A drawback of the aforementioned predictive coding is that the availability of the IBC prediction mode both at the encoder and at the decoder increases the calculational resources. Moreover, at the encoder, it is necessary to code a syntax element specific to the IBC prediction mode and to write it into the signal to be transmitted to the decoder, thereby increasing the signaling cost. Finally, the IBC prediction mode involves the storage of all the image data at the encoder and at the decoder, so as to provide to be used for the coding and the decoding of the current block.

The invention is therefore aimed at a coder/decoder which proposes to apply at a current instant two different prediction modes to a current image arising from a video content of a given type or format, while making it possible to optimize:

-   -   the coding/decoding performance for the current image,     -   the complexity of the coding/decoding of the current image,     -   the reduction in the memory resources at the encoder and at the         decoder.

SUBJECT AND SUMMARY OF THE INVENTION

One of the aims of the invention is to remedy drawbacks of the aforementioned prior art.

To this effect, a subject of the present invention relates to a method for coding an image split up into blocks, said image containing two distinct zones.

Such a coding method is noteworthy in that it implements the following:

-   -   solely in the case where the current block belongs to one of the         zones of the image:         -   code the current block with the aid of a prediction, the             current block being predicted by using a previously coded             and then decoded block situated in the other zone of the             image, the blocks of the other zone having been previously             coded and then decoded,         -   code an item of information which indicates the application             of the prediction used,     -   for any decoded block which belongs to the other zone of the         image, store the decoded-block data.

Such a provision makes it possible to reduce the complexity of the coding since a particular prediction mode is not used for the other zone of the image and will not therefore be set into competition with respect to other prediction modes during the coding of a block situated in the other zone.

Such a provision makes it possible furthermore to reduce the costs of storage of the coding data, since the data of the coded and then decoded blocks of the image are stored only for one of the two zones of the image.

According to a particular embodiment, the coding method comprises the coding of at least one item of location information in respect of the previously coded and then decoded block. Such an item of location information can be a motion vector or a set of coordinates.

Said item of location information thus coded is transmitted to the decoder, thereby allowing to it, during the decoding of a current block belonging to one of the zones of the current image, to locate the decoded block situated in the other zone of the current image.

According to a particular embodiment, the coding of an item of identification information in respect of each of the two zones of the image is implemented.

Such a provision makes it possible to transmit to the decoder the position of each of the zones with respect to one another.

Correlatively, the invention relates to a device for coding an image split up into blocks, said image containing two distinct zones, comprising a processing circuit which is designed to:

-   -   solely in the case where the current block belongs to one of the         zones of the image:         -   code the current block with the aid of a prediction, the             current block being predicted by using a previously coded             and then decoded block situated in the other zone of the             image, the blocks of the other zone having been previously             coded and then decoded,         -   code an item of information which indicates the application             of the prediction used,     -   for any decoded block which belongs to the other zone of the         image, store the decoded-block data.

In a corresponding manner, the invention also relates to a method for decoding a data signal representative of an image split up into blocks which has been coded, said at least one image containing two distinct zones.

Such a decoding method is noteworthy in that it implements the following:

-   -   solely in the case where the current block belongs to one of the         zones of the image:         -   read in the data signal an item of information which             indicates the decoding of the current block with the aid of             a prediction, the current block being intended to be             predicted by using a previously decoded block, situated in             the other zone of the image, the blocks of said other zone             having been previously decoded,         -   decode the current block by using said prediction,     -   for any decoded block which belongs to the other of said zones         of the image, store the decoded-block data.

According to a particular embodiment, the decoding method comprises the reading, in the data signal, of at least one item of location information in respect of the previously coded and then decoded block.

According to a particular embodiment, each of the zones of the image is identified by reading a corresponding item of identification information read in the data signal.

Correlatively, the invention relates to a device for decoding a data signal representative of an image split up into blocks which has been coded, said image containing two distinct zones, comprising a processing circuit which is designed to:

-   -   solely in the case where the current block belongs to one of the         zones of the image:         -   read in the data signal an item of information which             indicates the decoding of the current block with the aid of             a prediction, the current block being intended to be             predicted by using a previously decoded block, situated in             the other zone of the image, the blocks of said other zone             having been previously decoded,         -   decode the current block by using said prediction,     -   for any decoded block which belongs to the other of said zones         of the image, store the decoded-block data.

The invention further relates to a computer program comprising instructions for implementing one of the coding and decoding methods according to the invention, when it is executed on a computer.

Such a program can use any programming language, and be in the form of source code, object code, or of code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form.

Yet another subject of the invention also envisages a recording medium readable by a computer, and comprising computer program instructions, such as mentioned hereinabove.

The recording medium can be any entity or device capable of storing the program. For example, the medium can comprise a storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or else a magnetic recording means, a digital recording means, for example a USB key or a hard disk.

Moreover, such a recording medium can be a transmissible medium such as an electrical or optical signal, which can be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular be downloaded over a network of Internet type.

Alternatively, such a recording medium can be an integrated circuit in which the program is incorporated, the circuit being adapted to execute the method in question or to be used in the execution of the latter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages will become apparent on reading preferred embodiments described with reference to the figures in which:

FIG. 1 represents the progress of the coding method according to a first embodiment of the invention,

FIG. 2 represents a coding device according to an embodiment of the invention,

FIGS. 3A to 3C each represent examples of distinct zones in the current image,

FIG. 4 represents the progress of the decoding method according to an embodiment of the invention,

FIG. 5 represents a decoding device according to an embodiment of the invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Detailed Description of the Coding Part

An embodiment of the invention will now be described, in which the coding method according to the invention is used to code an image or a sequence of images according to a binary stream close to that obtained by a coding implemented in a coder complying with any one of the current or forthcoming video coding standards.

In this embodiment, the coding method according to the invention is for example implemented in a software or hardware manner by modifications of such a coder. The coding method according to the first embodiment of the invention is represented in the form of an algorithm comprising operations C1 to C100 a) or C1 to C101 b) such as are represented in FIG. 1 .

According to the embodiment of the invention, the coding method according to the invention is implemented in a coding device or coder CO represented in FIG. 2 .

As illustrated in FIG. 2 , the coder CO comprises a memory MEM_C comprising a buffer memory MT_C, a processor PROC_C driven by a computer program PG_C which implements the coding method according to the invention. On initialization, the code instructions of the computer program PG_C are for example loaded into a RAM memory, denoted MR_C, before being executed by the processor PROC_C.

The coding method represented in FIG. 1 applies to any current image IC_(j) which is fixed or else which forms part of a sequence of L images IC₁, . . . , IC_(j), . . . , IC_(L) (1≤j≤L) to be coded.

The current image IC_(j) arises from at least one video sequence comprising, by way of non-exhaustive examples:

-   -   images arising from one and the same camera and following one         another temporally (coding/decoding of 2D type),     -   images arising from various cameras oriented according to         different views (coding/decoding of 3D type),     -   corresponding texture components and depth components, that is         to say which are representative of one and the same scene         (coding/decoding of 3D type),     -   images obtained by projection of a 360° video,     -   images obtained by projection of a stereo video and each         comprising two views representative of one and the same scene,         which are intended to be looked at by the left eye and the right         eye of the user, respectively,     -   non-natural images of the “screen Content” type, such as for         example images obtained by screen video capture,     -   etc. . . . .

With reference to FIG. 1 , there is undertaken at C1, in a manner known per se, the splitting of a current image IC_(j) into a plurality of blocks B₁, B₂, . . . , B_(u), . . . , B_(S) (1≤u≤S). The partitioning is implemented by a partitioning device MP_C represented in FIG. 2 , which device is driven by the processor PROC_C.

It should be noted that within the meaning of the invention, the term “block” signifies coding unit. The latter terminology is used notably in the HEVC standard “ISO/IEC/23008-2 Recommendation ITU-T H.265 High Efficiency Video Coding (HEVC)”.

In particular, such a coding unit groups together sets of pixels of rectangular or square shape, also called blocks, macroblocks, or else sets of pixels exhibiting other geometric shapes.

Said blocks B₁, B₂, . . . , B_(u), . . . , B_(S) are intended to be coded according to a predetermined order of traversal, which is for example of the lexicographic type. This signifies that the blocks are coded one after the other, from left to right.

Other types of traversal are of course possible. Thus, it is possible to split the image IC_(j) into several sub-images called slices and to apply a splitting of this type independently to each sub-image. It is also possible to code not a succession of lines, as explained hereinabove, but a succession of columns. It is also possible to traverse the lines or columns in either direction.

According to an example, the blocks B₁, B₂, . . . , B_(u), . . . , B_(S) have a square shape and all contain K pixels, with K≥1. By way of non-exhaustive example, the blocks have a size of 64×64 pixels and/or 32×32 and/or 16×16 and/or 8×8 pixels.

As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks at the bottom might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not aligned with one another.

At C2, there is undertaken the coding of a first syntax element activate_ibc which is associated with a characteristic of the current image IC_(j).

The syntax element activate_ibc is a high-level syntax element of a video sequence comprising the current image IC_(j). To this effect, as a function of the coding context, this element can be coded:

-   -   at the start of the coding of each image of the video sequence,     -   or just once at the start of the coding of a sequence of images,     -   or just once at the start of the coding of the video sequence.

The syntax element activate_ibc is intended to indicate whether, for the current block to be coded, a prediction mode of the “Intra Block Copy” type mentioned above in the description, according to which the current block, situated in a first zone of the current image, is predicted with respect to a previously coded and then decoded block situated in a second zone of the current image, the blocks of the second zone having been previously coded and then decoded, is set into competition with other predetermined prediction modes. According to the invention, the first and second zones are distinct, in the sense that they do not overlap.

The coding C2 is for example an entropy coding of CABAC (“Context Adaptive Binary Arithmetic Coder”) type or else an entropy coding of arithmetic or Huffman type. This coding is implemented by a coding device MC_C represented in FIG. 2 , which device is driven by the processor PROC_C.

According to a preferred embodiment, the syntax element activate_ibc takes the value 0 if the prediction mode of the “Intra Block Copy” type is not set into competition and the value 1 if the prediction mode of the “Intra Block Copy” type is set into competition.

According to a first embodiment represented in FIG. 3A, a current image IC_(j) comprises a first zone Z1 which extends over the bottom half of the image and a second zone Z2 which extends over the top half of the image.

According to a second embodiment represented in FIG. 3B, a current image IC_(j) comprises a first zone Z1 which extends over the right half of the image and a second zone Z2 which extends over the left half of the image.

According to a third embodiment represented in FIG. 3C, a current image IC_(j) comprises a first zone Z1 which extends over the bottom left quarter of the image and a second zone Z2 which extends over the top left quarter of the image.

Other configurations are of course possible. For example, the zones Z1 and Z2 may be swapped. Furthermore the zones Z1 and Z2 may or may not have the same shape.

Optionally, as represented dashed in FIG. 1 , there is undertaken, at C3, the coding of a second syntax element fp_type which is associated with another characteristic of the current image IC_(j).

The syntax element fp_type is also a high-level syntax element of a video sequence comprising the current image IC_(j). To this effect, as a function of the coding context, this element can be coded:

-   -   at the start of the coding of each image of the video sequence,     -   or just once at the start of the coding of a sequence of images,     -   or just once at the start of the coding of the video sequence.

The syntax element fp_type is intended to indicate if the current image is composed of several views captured at the same temporal instant and arranged in the current image so as to form a single view (rectangle of pixels). A method for composing such an image uses for example the technique called “Frame Packing” (FP).

The coding C3 is for example an entropy coding of CABAC type or else an entropy coding of arithmetic or Huffman type. This coding is implemented by the coding device MC_C of FIG. 2 .

According to a preferred embodiment, the syntax element fp_type takes for example:

-   -   the value 01 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3A,     -   the value 10 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3B,     -   the value 11 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3C.

With reference to FIG. 1 , the coder CO of FIG. 2 selects at C4 a current block to be coded B_(u) of the image IC_(j).

At C5, there is undertaken the location of the current block B_(u) of the image IC_(j) for example by determining its coordinates with respect to the first pixel situated at the top left of the image IC_(j), and which has coordinates (0,0).

The location C5 is implemented by a calculation device CAL1_C such as represented in FIG. 2 , which device is driven by the processor PROC_C.

If the current block B_(u) belongs to the first zone Z1 of the image IC_(j) and if the first syntax element activate_ibc has the value 1, there is undertaken at C6 a) the prediction of the current block B_(u) by the prediction technique of the “Intra Block Copy” type, as well as by conventional, for example Intra and/or Inter, prediction techniques. To this effect, the block B_(u) is predicted with respect to at least one predictor block in accordance with a mode of prediction belonging to one of the prediction techniques which have just been mentioned. In the course of the prediction C6 a), at least one item of location information in respect of the predictor block is determined. Such an item of information is for example, a motion vector which points at the predictor block, the coordinates of the first pixel at the top left of the predictor block, etc. . . . .

In a manner known per se, the current block B_(u) is predicted with respect to a plurality of candidate predictor blocks. Each of the candidate predictor blocks is a block of pixels which has been already coded and then decoded.

Thus, in the case of the intra prediction technique, the current block B_(u) is predicted by interpolation on the basis of values of decoded pixels of candidate predictor blocks which are situated immediately to the left of or above the current block.

In the case of the inter prediction technique, the current block B_(u) is predicted by interpolation on the basis of values of decoded pixels of candidate predictor blocks which are situated in one or more previously processed and stored images.

In accordance with the invention, in the case of the “Intra Block Copy” prediction technique, the current block B_(u) is predicted by interpolation on the basis of values of decoded pixels of candidate predictor blocks which are situated in the second zone Z2 of the current image IC_(j).

On completion of the prediction step C6 a), an optimal predictor block BP_(opt) is obtained subsequent to a setting into competition of said predetermined prediction techniques, according to a predetermined coding performance criterion, for example by minimizing a distortion bitrate criterion well known to the person skilled in the art. The block BP_(opt) is considered to be an approximation of the current block B_(u). The information relating to this prediction is intended to be written, in the form of syntax elements, into a data signal or stream to be transmitted to a decoder.

In accordance with the invention:

-   -   in the case where the block BP_(opt) has been obtained by “Intra         Block Copy” prediction, there is undertaken at C100 a) the         coding of a syntax element ibc_flag by the coding device MC_C of         FIG. 2 . The syntax element ibc_flag is for example coded to the         value 1. Other syntax elements indicate the location of the         predictor block BP_(opt) stored in the second zone Z2 of the         current image IC_(j): it entails for example a motion vector         which points at the predictor block BP_(opt), the coordinates of         the first pixel at the top left of the predictor block BP_(opt),         etc . . . ;         -   in the case where the block BP_(opt) has not been obtained             by “Intra Block Copy” prediction, the syntax element             ibc_flag is moreover coded to the value 0.

There is thereafter undertaken at C7 a) the comparison of the data relating to the current block B_(u) with the data of the predictor block BP_(opt). Such a comparison consists in calculating the difference between the predictor block obtained BP_(opt) and the current block B_(u).

A data set, called residual block Br_(u), is then obtained.

The operations C6 a) and C7 a) are implemented by a predictive coding device PRED_C represented in FIG. 2 , which device is driven by the processor PROC_C.

With reference again to FIG. 1 , the data of the current residual block Br_(u) are coded at C8 a).

According to a nonlimiting exemplary embodiment, such a coding C8 a) implements the application C81 a) of a transform to the pixels of the current residual block Br_(u).

In a manner known per se, as a function of the context or of the coding standard used, such a transform is for example a transform of DCT (English abbreviation of “Discrete Cosine Transform”), DST (English abbreviation of “Discrete Sine Transform”) type, of DWT (English abbreviation of “Discrete Wavelet Transform”) type or else of LT (English abbreviation of “Lapped Transform”) type. These transforms are stored previously in a list LTS, in the buffer memory MT_C of the coder CO of FIG. 2 .

On completion of the application of this transform, a current transformed data block Btu is obtained.

Such an operation is performed by a transform calculation device MTR_C, such as represented in FIG. 2 , which device is driven by the processor PROC_C.

The coding C8 a) furthermore implements a quantization C82 a) of the data of the transformed block Btu according to a conventional quantization operation, such as for example a scalar or vector quantization. A block Bq_(u) of quantized coefficients is then obtained.

The quantization C82 a) is implemented by a quantization device MQ_C such as represented in FIG. 2 , which device is driven by the processor PROC_C.

The transform calculation device MTR_C and the quantization device MQ_C are contained in a device for coding blocks MCB_C which is represented in FIG. 2 , which device is driven by the processor PROC_C.

The coding C8 a) furthermore implements a coding C83 a) of the data of the block Bq_(u) of quantized coefficients. The coding C83 a) is implemented by the coding device MC_C of FIG. 2 . A set of coded data DC_(u) of the current block B_(u) is obtained on completion of the coding C83 a).

With reference to FIG. 1 , there is undertaken at C9 a) the construction of a portion of a data signal F, said portion containing in particular according to the invention:

-   -   the coded data DC_(u) obtained at C8 a),     -   the value 0/1 of the syntax element ibc_flag,     -   optionally the value 0/1 of the first syntax element         activate_ibc, if the latter is coded at the image level,     -   optionally the value 0/1 of the second syntax element fp_type,         if the latter is coded at the image level.

The construction of the signal portion F is implemented by a data signal construction device MCF, such as represented in FIG. 2 .

If on completion of the location C5, the current block B_(u) belongs to the second zone Z2 of the image IC_(j) and if the first syntax element activate_ibc has the value 1, there is undertaken at C6 b) the prediction of the current block B_(u) by conventional, such as for example Intra and/or Inter, prediction techniques. To this effect, the block B_(u) is predicted with respect to at least one predictor block in accordance with a prediction mode belonging to one of the conventional prediction techniques which have just been mentioned.

On completion of prediction step C6 b), an optimal predictor block BP_(opt) is obtained subsequent to a setting into competition of said predetermined conventional prediction techniques, according to a predetermined coding performance criterion, for example by minimization of the distortion bitrate criterion. The block BP_(opt) is considered to be an approximation of the current block B_(u). The information relating to this prediction is intended to be written, in the form of syntax elements, into a data signal or stream to be transmitted to a decoder.

At C7 b) a calculation of the difference between the predictor block obtained BP_(opt) and the current block B_(u) is implemented conventionally.

A data set, called residual block Br_(u), is then obtained.

A coding C8 b) of the residual block Br_(u) is implemented and comprises operations C81 b), C82 b) and C83 b) which are respectively identical to the aforementioned operations C81 a), C82 a) and C83 a) and which for this reason, are not described again here.

On completion of the coding C83 b), a set of coded data DC_(u) of the current block B_(u) is obtained.

With reference to FIG. 1 , there is undertaken at C9 b) the construction of a portion of the data signal F, said portion containing the coded data DC_(u) obtained at C8 b) and information relating to the prediction of the current block used. It should be noted that the portion of the signal F does not contain the value 0 or 1 of the syntax element ibc_flag, the “Intra Block Copy” prediction not having been set into competition during the coding of the current block of the second zone Z2. The stream F contains:

-   -   optionally the value 0/1 of the first syntax element         activate_ibc, if the latter is coded at the image level,     -   optionally the value 0/1 of the second syntax element fp_type,         if the latter is coded at the image level

In accordance with the invention, if first syntax element activate_ibc has the value 1, there is undertaken at C100 b) the calculation of a decoded version BD_(u) of the current block B_(u) on the basis of the coded data set DC_(u) obtained on completion of the coding C8 b). Such a decoded version corresponds to the decoded block obtained on completion of the method, which will be described further on in the description, for decoding the image IC_(j).

The calculation C100 b) is implemented by a calculation device CAL2_C such as represented in FIG. 2 , which device is driven by the processor PROC_C.

The decoded version BD_(u) of the current block B_(u) is stored at C101 b) in a space ES of the buffer memory MT_C of the coder CO of FIG. 2 so as to be used by the coder CO to predict the following blocks of the current image IC_(j) or of another image.

The calculation C100 b) and the storage C101 b) are implemented either before the construction C9 b) of the signal portion F, or after.

In a manner known per se, irrespective of the construction C9 a) or C9 b) of signal portion F implemented, the signal portion F can furthermore comprise certain information encoded by the coder CO, such as for example:

-   -   the Inter and/or Intra prediction type if the current block         B_(u) has been inter- and/or intra-predicted, and if relevant,         the prediction mode selected, the index of the predictor block         obtained,     -   the type of partitioning of the current block B_(u) if the         latter has been partitioned,     -   the type of transform applied to the data of the current block         B_(u),     -   etc. . . . .

The data signal F is thereafter transmitted by a communication network (not represented) to a remote terminal. The latter comprises the decoder DO represented in FIG. 7 .

The coding operations C1 to C100 a) or else C1 to C101 b) which have just been described hereinabove are thereafter implemented for each of the blocks B₁, B₂, . . . , B_(u), . . . , B_(S) to be coded of the current image IC_(j) considered, in a differentiated manner depending on whether the current block is situated in the first zone Z1 or the second zone Z2 of the current image IC_(j).

Detailed Description of the Decoding Part

An embodiment of the invention will now be described, in which the decoding method according to the invention is used to decode a data signal or stream representative of an image or of a sequence of images which is able to be decoded by a decoder complying with any one of the current or forthcoming video decoding standards.

In this embodiment, the decoding method according to the invention is for example implemented in a software or hardware manner by modifications of such a decoder.

The decoding method according to the invention is represented in the form of an algorithm comprising operations D1 to D11 a) or D1 to D100 b) such as represented in FIG. 4 .

According to this embodiment, the decoding method according to the invention is implemented in a decoding device or decoder DO represented in FIG. 5 .

As illustrated in FIG. 5 , according to this embodiment of the invention, the decoder DO comprises a memory MEM_D which itself comprises a buffer memory MT_D, a processor PROC_D driven by a computer program PG_D which implements the decoding method according to the invention. On initialization, the code instructions of the computer program PG_D are for example loaded into a RAM memory denoted RAM_D, before being executed by the processor PROC_D.

The decoding method represented in FIG. 4 applies to any coded current image IC_(j) which is fixed or else which forms part of a sequence of L images IC₁, . . . , IC_(j), . . . , IC_(L) (1≤j≤L) to be decoded.

The current image IC_(j) to be decoded arises from at least one video sequence comprising, by way of non-exhaustive examples:

-   -   images arising from one and the same camera and following one         another temporally (coding/decoding of 2D type),     -   images arising from various cameras oriented according to         different views (coding/decoding of 3D type),     -   corresponding texture components and depth components, that is         to say which are representative of one and the same scene         (coding/decoding of 3D type),     -   images obtained by projection of a 360° video,     -   images obtained by projection of a stereo video and each         comprising two views representative of one and the same scene,         which are intended to be looked at respectively through the left         eye and the right eye of the user,     -   non-natural images of the “screen Content” type, such as for         example the images obtained by screen video capture,     -   etc. . . . .

At D1, there is undertaken the reading, in the data signal F, of the coded value 0 or 1 of the first syntax element activate_ibc which is associated with a characteristic of the current image IC_(j) to be decoded, in the case where this syntax element has been coded at the level of the current image IC_(j). As the identification D1 is optional, it is represented dashed in FIG. 4 .

At D2, there is undertaken the reading, in the data signal F, of the coded value 0 or 1 of the second syntax element fp_type which is associated with a characteristic of the current image IC_(j) to be decoded, in the case where this syntax element has been coded at the level of the current image IC_(j). As the identification D2 is optional, it is represented dashed in FIG. 4 .

According to a preferred embodiment, the syntax element fp_type which is read takes for example:

-   -   the value 01 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3A,     -   the value 10 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3B,     -   the value 11 to indicate that the zones Z1 and Z2 are arranged         as represented in FIG. 3C.

The identifications D1 and D2 are implemented by a stream analysis device PARS_D, such as represented in FIG. 5 , said device being driven by the processor PROC_D.

In case of identification, at D3, the coded value 0 or 1 taken by the first syntax element activate_ibc and/or one of the values 01, 10, 11 taken by the second syntax element fp_type are decoded.

Such a decoding D3 is implemented by a decoding device MD_D represented in FIG. 5 , which device is driven by the processor PROC_D.

The decoding is for example an entropy decoding of CABAC type or else an entropy decoding of arithmetic or Huffman type.

With reference to FIG. 4 , there is undertaken at D4 the identification, in the signal F, of the coded data DC₁, DC₂, . . . , DC_(u), . . . , DC_(S) (1≤u≤S) associated respectively with the blocks B₁, B₂, . . . , B_(u), . . . , B_(S) coded previously in accordance with the aforementioned lexicographic order, which have been obtained on completion of the coding operation C8 a) or C8 b) of FIG. 1 . This signifies that the blocks are decoded one after another, from left to right, in a corresponding manner to the order of coding mentioned above.

Such an identification D4 is implemented by the stream analysis device PARS_D of FIG. 4 .

Other types of traversal than that which has just been described hereinabove are of course possible and depend on the order of traversal chosen on coding.

According to an example, the blocks B₁, B₂, . . . , B_(u), . . . , B_(S) have a square shape and all contain K pixels, with K≥1. By way of non-exhaustive example, the blocks have a size of 64×64 pixels and/or 32×32 and/or 16×16 and/or 8×8 pixels.

As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks at the bottom might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not aligned with one another.

With reference to FIG. 4 , at D5, the decoder DO of FIG. 5 selects as current block to be decoded a current set of coded data DC_(u) of the image IC_(j), which set is associated with a block B_(u) to be decoded.

At D6, there is undertaken the location of the current block B_(u) to be decoded of the image IC_(j), for example by determining its coordinates with respect to the first reconstructed pixel of the image IC_(j), which pixel is situated at the top left of the latter and has coordinates (0,0).

The location D6 is implemented by a calculation device CAL_D such as represented in FIG. 5 , which device is driven by the processor PROC_D.

If the current block B_(u) belongs to the first zone Z1 of the image IC_(j), and if the first syntax element activate_ibc decoded at D1 has the value 1, there is undertaken at D7 a) the reading, in the data signal F, of the syntax element ibc_flag, by means of the stream analysis device PARS_D of FIG. 5 , and of its decoding, by means of the decoding device MD_D.

According to the invention, if the first syntax element activate_ibc decoded at D1 or else previously has the value 1, there is undertaken at D6 a) the reading in the data signal F of the syntax element ibc_flag, by means of the stream analysis device PARS_D of FIG. 5 , and of its decoding, by means of the decoding device MD_D.

In accordance with the invention:

-   -   if the syntax element ibc_flag has the value 1, there is         implemented at D8 a) a determination of the predictor block         BP_(opt) serving in the prediction of the “Intra Block Copy”         type for the block B_(u) to be decoded, the location of the         predictor block BP_(opt) stored in the second zone of the image         IC_(j) being identified by syntax elements which are decoded in         the data signal F: such syntax elements are for example a motion         vector which points at the predictor block BP_(opt), the         coordinates of the first pixel at the top left of the predictor         block BP_(opt), etc . . . ;     -   if the syntax element ibc_flag has the value 0, there is         implemented at D8 a) a determination of the predictor block         BP_(opt) serving in the prediction of the current block B_(u)         which is different from the prediction of the “Intra Block Copy”         type. It may entail for example an intra or inter conventional         prediction on completion of which a predictor block BP_(opt)         associated with an indicator previously read in the data signal         F is obtained.

With reference to FIG. 4 , the coded data DC_(u) of the current block B_(u) are decoded at D9 a). Such a decoding is implemented by a device for decoding blocks MDB_D which is represented in FIG. 5 , which device is driven by the processor PROC_D.

The decoding D9 a) implements, at D91 a), a decoding of the data DC_(u) associated with the current block B_(u) to be decoded and which have been coded at C8 a) in FIG. 1 . On completion of such a decoding, a set of numerical information is obtained, the latter being associated with the block of quantized coefficients Bq_(u) which was obtained at C82 a) in FIG. 1 .

The decoding D91 a) is implemented by the decoding device MD_D represented in FIG. 5 .

The decoding D9 a) furthermore implements a dequantization D92 a) of the block of quantized coefficients Bq_(u), according to a conventional dequantization operation which is the operation inverse to the quantization C82 a) of FIG. 1 . A current set of dequantized coefficients BDq_(u) is then obtained. Such a dequantization is for example of scalar or vector type and is implemented by means of an inverse quantization device MQ⁻¹_D, such as represented in FIG. 1 , which device is driven by the processor PROC_D.

The decoding D9 a) furthermore implements the application D93 a) of a transform to the current set of dequantized coefficients BDq_(u) obtained at D92 a). In a manner known per se, such a transform is a transform inverse to that applied to the coding at C81 a) in FIG. 1 , such as for example a DCT, DST, DWT, LT or other transform. In a corresponding manner to the coder CO of FIG. 2 , these transforms form part of a list of transforms LTS⁻¹ which is stored previously in the buffer memory MT_D of the decoder DO of FIG. 5 . The type of transform to be applied can be determined at the decoder conventionally, by reading, in the data signal F, the index of the transform applied to the coding.

The transform application D93 a) is performed by a transform calculation device MTR⁻¹_D, such as represented in FIG. 5 , which device is driven by the processor PROC_D.

The inverse quantization device MQ⁻¹_D and the transform calculation device MTR⁻¹_D are contained in a device for decoding blocks MDB_D which is represented in FIG. 5 , which device is driven by the processor PROC_D.

A current decoded residual block BDr_(u) is obtained on completion of the decoding D9 a) of the data of the current block.

With reference to FIG. 4 , at D10 a), the current decoded residual block BDr_(u) is added to the predictor block BP_(opt) located at D6 a).

The operation D10 a) is implemented by a predictive decoding device PRED_D represented in FIG. 5 , which device is driven by the processor PROC_D.

With reference to FIG. 4 , there is undertaken at D11 a) the writing of the current decoded block BD_(u) in a decoded image ID_(j).

The writing D11 a) is implemented by an image reconstruction device URI such as represented in FIG. 5 , the device URI being driven by the processor PROC_D.

On completion of the location D6 of FIG. 4 , if the current block B_(u) to be decoded belongs to the second zone Z2 of the image IC_(j), there is undertaken at D7 b):

-   -   the reading in the data signal F, by means of the stream         analysis device PARS_D of FIG. 5 , of information relating to         the conventional prediction C6 b) implemented on coding,     -   and of the decoding of this information, by means of the         decoding device MD_D.

With reference to FIG. 4 , there is implemented at D8 b) a determination of the predictor block BP_(opt) serving in the prediction of the current block B_(u) which is different from the prediction of the “Intra Block Copy” type. It may entail for example an intra or inter conventional prediction on completion of which is obtained a predictor block BP_(opt) associated with the information which is read at C6 b) in the data signal F, and is then decoded.

With reference to FIG. 4 , the coded data DC_(u) of the current block B_(u) are decoded at D9 b) by the device for decoding blocks MDB_D of FIG. 5 .

The decoding D9 b) comprises operations D91 b), D92 b) and D93 b) which are respectively identical to the aforementioned operations D91 a), D92 a) and D93 a) and, which for this reason, are not described again here.

On completion of the decoding D9 b) of the data of the current block, a current decoded residual block BDr_(u) is obtained.

With reference to FIG. 4 , at D10 b), the predictive decoding device PRED_D of FIG. 5 adds the current decoded residual block BDr_(u) to the predictor block BP_(opt) located at D6.

With reference to FIG. 4 , there is undertaken at D11 b) the writing of the current decoded block BD_(u) in a decoded image ID_(j).

In accordance with the invention, if first syntax element activate_ibc has the value 1, with reference to FIG. 4 , there is undertaken at D100 b) the storage of the current decoded block BD_(u) in a space ES of the buffer memory MT_D of the decoder DO of FIG. 5 so as to be used by the decoder DO to predict the following blocks of the current image IC_(j) to be decoded or of another image.

The storage D100 b) is implemented either before the operation of writing D11 b) the current decoded block in the decoded image ID_(j), or after.

The decoding method which has just been described hereinabove is implemented for all the coded data of blocks DC₁, DC₂, . . . , DC_(u), . . . , DC_(S) wherein said data are associated respectively with the blocks B₁, B₂, . . . , B_(u), . . . , B_(S) to be decoded of the current image IC_(j) considered and in a differentiated manner depending on whether the coded data are associated with a block situated in the first zone or the second zone of the current image IC_(j) to be decoded.

It goes without saying that the embodiments which have been described hereinabove have been given purely by way of wholly non-limiting indication, and that numerous modifications can easily be made by the person skilled in the art without however departing from the scope of the invention. 

What is claimed is:
 1. A coding method comprising: coding a current image split up into blocks, said current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the coding of any block of said current image, wherein said current image has been obtained by projection of a 360° video and wherein said coding comprises the following acts performed by a coding device: coding a current block of said first zone, by: predicting the current block by using a previously coded and then decoded block situated in said second zone, and coding an item of information which indicates if a mode of prediction Intra Block Copy has been used or not to predict the current block; decoding the coded current block of the first zone; and storing data of the decoded current block as a function of the item of information that has been coded.
 2. The coding method as claimed in claim 1, comprising coding at least one item of location information in respect of the previously coded and then decoded block.
 3. The coding method as claimed in claim 1, implementing coding an item of identification information in respect of each of the at least two zones of the current image.
 4. A coding device comprising: a processing circuit which is designed to code a current image split up into blocks, said current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the coding of any block of said current image, wherein said current image has been obtained by projection of a 360° video and wherein the processing circuit codes the current image by: coding a current block of said first zone, by: predicting the current block by using a previously coded and then decoded block situated in said second zone, and coding an item of information which indicates if a mode of prediction Intra Block Copy has been used or not to predict the current block; decoding the coded current block of the first zone; and storing data of the decoded current block as a function of the item of information that has been coded.
 5. A non-transitory computer-readable recording medium on which is recorded a computer program comprising program code instructions which when executed by a processor of a coding device configure the coding device to: code a current image split up into blocks, said current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the coding of any block of said current image, wherein said current image has been obtained by projection of a 360° video and wherein the coding comprises: code a current block of said first zone, by: predicting the current block by using a previously coded and then decoded block situated in said second zone, and coding an item of information which indicates if a mode of prediction Intra Block Copy has been used or not to predict the current block; decode the coded current block of the first zone; and store data of the decoded current block as a function of the item of information that has been coded.
 6. A decoding method comprising: decoding a data signal representative of a current image split up into blocks which has been coded, said at least one current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the decoding of any coded block of said data signal, wherein said current image has been obtained by projection of a 360° video and wherein said decoding comprises the following acts performed by a decoding device: decoding a current block of said first zone, by: reading in the data signal an item of information which indicates if a mode of prediction Intra Block Copy is to be used or not to predict the current block, the current block being intended to be predicted by using a previously decoded block, situated in said second zone, and decoding the current block by using a prediction indicated by said read item of information; and storing data of the decoded current block as a function of the item of information that has been read.
 7. The decoding method as claimed in claim 6, comprising reading, in the data signal, at least one item of location information in respect of the previously coded and then decoded block.
 8. The decoding method as claimed in claim 6, in which each of the zones of the current image is identified by reading a corresponding item of identification information read in the data signal.
 9. A decoding device comprising: a processing circuit which is designed to decode a data signal representative of a current image split up into blocks which has been coded, said current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the decoding of any block of said data signal, wherein said current image has been obtained by projection of a 360° video and wherein the processing circuit decodes the data signal by: decoding a current block of said first zone, by: reading in the data signal an item of information which indicates if a mode of prediction Intra Block Copy is to be used or not to predict the current block, the current block being intended to be predicted by using a previously decoded block, situated in said second zone, and decoding the current block by using a prediction indicated by said read item of information; and storing data of the decoded current block as a function of the item of information that has been read.
 10. A non-transitory computer-readable recording medium on which is recorded a computer program comprising program code instructions which when executed by a processor of a decoding device configure the decoding device to: decode a data signal representative of a current image split up into blocks which has been coded, said current image containing at least two distinct first and second zones of blocks which do not overlap and are defined previously to the decoding of any block of the data signal, wherein said current image has been obtained by projection of a 360° video and wherein the decoding comprises: decode a current block of said first zone, by: reading in the data signal an item of information which indicates if a mode of prediction Intra Block Copy is to be used or not to predict to predict the current block, the current block being intended to be predicted by using a previously decoded block, situated in said second zone, and decoding the current block by using a prediction indicated by said read item of information; and store data of the decoded block as a function of the item of information that has been read. 